Overlay target design method minimize impact of lens aberrations

ABSTRACT

A method of designing an alignment target system to minimize lens aberrations is disclosed. A first layer alignment target&#39;s pitch is selected based on the minimum feature size of the circuit. The second layer alignment target&#39;s pitch is selected based on the diffraction pattern of the first layer&#39;s target and the illumination settings of the second layer. Displacement errors are minimized when the second layer target&#39;s 1 st  diffraction order overlaps the first layer target&#39;s 0 th  diffraction order.

BACKGROUND OF THE INVENTION

[0001] The invention relates to an overlay target design method forsemiconductor fabrication to minimize the impact of lens aberrations ontarget projection.

DESCRIPTION OF RELATED ART

[0002] Typically semiconductor devices are fabricated by opticallithography techniques using a projection imaging systems. A typicalprojection image system 20 is illustrated in FIG. 1. The system 20 at aminimum includes an illumination controller 22 and an illuminationsource 24 coupled with and controlled by controller 22. Illuminationsource 24 may include, for example, a mirror, a lamp, a light filter,and a condenser lens system. As used herein, the term “light” refers tolight used in photolithography. The term “light” need not be restrictedto visible light, but may also include other forms of radiation andlithography. For example, energy supplied by lasers, photons, ion beams,electron beams, or X-rays are included within the term “light”.

[0003] Illumination source 24 emits light or radiation that can passthrough openings in mask 26. System 20 shows mask 26 positioned adjacentto light source 24; optionally, other devices such as one or moreoptical lenses could separate light source 24 and mask 26. The term“mask” is not limited to a physical structure, but also includes adigitized image used in, for example, electron beam and ion beamlithography systems. For example, mask 26 may include a pattern forprojecting a wiring or feature pattern of an integrated circuit. Thepattern of mask 26 may include various image structures, for example,clear areas, opaque areas, phase shifting areas, and overlay targets.Mask 26 generally is a combination of clear areas and opaque areas,where the clear areas allow light from light source 24 to pass throughmask 26 to form the mask's image. Light passing through mask 26 isfurther transmitted by projection lens 30, which may be, for example, areduction lens or a combination of lenses for focusing the mask patternonto a projection surface 111, such as a semiconductor wafer coveredwith a photoresist layer. Typical semiconductor fabrication involves afour to ten times reduction of mask size 26 by projection lens 30.Projection surface 111 is held in position by a holding device (notshown), which may be part of or controlled by a stepper (not shown).Also shown in FIG. 1 is lens pupil 28 of projection imaging lens 30,which defines the numerical aperture of lens 30.

[0004] As the dimension of features on integrated circuits continue todecrease, the resolution limits of optical lithography are quickly beingreached. One limit is caused by lens aberration, which is the failure ofa lens, such as projection lens 30, to produce exact point-to-pointcorrespondence between a received image, such as from mask 26, and aprojection surface 111, such as a semiconductor die 100 a portion ofwhich is illustrated in FIG. 2. One of the many types of lensaberrations in semiconductor device fabrication is coma aberrationswhich are optical aberrations that cause the image of a mask 26 toappear comet-shaped or blurred on die surface 113 (FIG. 2). Comaaberrations result in not only line width variations and/or patternasymmetry, but also affect the location or placement of the mask imageon the die surface 113. As discussed in Takashi Saito and HisashiWatanabe's article “Investigation of New Overlay Measurement Marks forOptical Lithography,” J. Vac Sci Technol. B 16(6), November/December1998, pp. 3415, 3418, during sub-micron device fabrication using opticallithography lens aberrations cause different displacement errors foroverlay targets and device patterns.

[0005] A target is a feature on a mask 26, usually at the perimeter ofthe mask 26, that is transferred to die surface 113 during theillumination phase. The target helps to determine if the image transferfrom mask 26 to die surface 113 was properly aligned relative to lowerlayers. Typically, the quality of the lithographic image alignment ismeasured by determining the alignment of a target on a lower level to atarget on an upper or overlay level. In general the image transfer issuccessful if the target of the upper-layer is approximately centeredwith the lower-level target. An, overlay measurement system is used tomeasure the distances and spaces between edges or boundaries of theupper and lower targets. It is critically important that the circuitpattern on one layer is accurately aligned with that of earlier layers.To evaluate the alignment of two layers, a target is formed on eachlayer. FIG. 2 is an illustration of a single integrated circuit (IC) die100 fabricated on a semiconductor wafer. The locations of the electricalcircuit or pattern and targets are represented by large box 120,hereinafter referred to as pattern, and small boxes 110, hereinafterreferred to as targets, respectively. Typical dimensions for die 100 are5 millimeters by 5 millimeters and typical dimensions for targets 110are 10 microns by 10 microns.

[0006] A prior art method of mask alignment measurement will bedescribed with respect to FIGS. 3 and 4. FIG. 3 is a top view of a priorart box-in-box target 110 of FIG. 2. FIG. 4 is a cross-sectional view ofFIG. 3 along line III-III. The accuracy of the transfer of the targets110 approximates the accuracy of the pattern 120 transfer. A firsttarget 112 with a box pattern can be formed on surface 113 in a firstlayer or under layer 115 using well known lithography techniques.Typically, a silicon oxide material is deposited over first layer 115 toform a second layer 116. The second target 114 is formed in second layer116 with dimensions smaller than target 112 using well known lithographytechniques. The perimeter 117 of first target 112 and perimeter 118 ofsecond target 114 can be viewed by optical measurement equipment. Bymeasuring the distance between the two perimeters or isolated edges 117,118 at several locations, the center positions of targets 112, 114 canbe determined and positional deviations between the two targets 112, 114can be determined. The overlay measurement provides a comparison of thealignment of the underlay target 112 of layer 115 with that of overlaytarget 114 of layer 116.

[0007] In the article by Saito and Watanabe the benefits of using finepattern targets made up of thin lines instead of large box shapedpatterns is discussed. Fine patterns targets, such as targets formedwith thin line widths, are generally much closer to the actual circuitfeatures dimensions than conventional large box patterns. Since lensaberrations typically induce line width variations and create alignmenterrors, using fine pattern targets allows more accurate detection oflens aberrations and alignment errors. In other words, the use of thetypical box-in-box method (FIGS. 3-4) to determine mask 26 displacementerrors is not very accurate for small device patterns, such as a quartermicron device fabrication (0.25 micron device feature size). Usingtargets with feature dimensions (size and pitch) similar to those of thecircuit improves the detection of displacement errors.

[0008] For example, FIG. 5 is an illustration of a conventional finepattern target system 200. Fine pattern targets 210, 220 are formed in afirst layer 215 (FIG. 6) over surface 213. Fine pattern targets 230, 240are formed in a second layer 216 (FIG. 6) over first layer 215. FIG. 6is a cross-sectional view of FIG. 5 along line VI-VI. In known targetsystems such as target system 200 (FIG. 5), the first layer targets 210,220 and second layer targets 230, 240 generally have the same pitch(P1). The term “pitch” refers to the distance between the outside edgeof a first target and the outside edge of a second target. For examplein FIGS. 5-6, the pitch of targets 210 and 220 are the distance betweenthe perimeter 211 of target 210 and the perimeter 212 of target 220. Inknown target systems 200 the pitches for targets in layers 215, 216 aregenerally the same. In addition, target line widths (W1) are generallythe same for targets in layers 215, 216. However even for targets in twodifferent layers 215, 216 with the same line width W1 and pitch P1,changes in the illumination settings of light source 24, such as wavelength, intensity, and annular size, used to form the targets 210, 220,230, 240 can cause misplacement of the second layer targets 230, 240 dueto projection lens 30 aberrations.

[0009] Illumination settings are an important design factor foroptimizing circuit feature dimensions, as the settings often change fromone layer to another. For example, if the second layer 216 targets 230,240 of FIGS. 5-6 are formed using different illumination settings thanthat used for first layer 215 targets 210, 220 the light will bediffracted differently by the mask 26. Since the target patterns havedifferent diffraction patterns, light will enter and exit lens 30(FIG. 1) at different locations. Due to normal variations in lenssurfaces, if light passes through different locations, lens aberrationswill cause the light to diffract differently causing target displacementin the second layer 216.

[0010] The displacement error is a function of the mechanical placementcapability of the system 20 and the projection lens 30 aberrations. Themechanical displacement is the same for both the pattern 120 and targets110. However, lens aberrations affect the pattern 120 and targets 110differently. In most cases, the lens induced error for the pattern 120is smaller than the lens induced error in typical box-in-box targets110. The lens error is more pronounced when different illuminationshapes are used on two different layers. Since the aberrations changeacross the lens 30 the light is subject to different aberrationpatterns. Hence corrections based on typical box-in-box targets 110induce displacement errors into the pattern 120.

[0011] There is a need and desire for a new method of designing featuredimensions, such as the pitch of second layer alignment targets, tominimize the impact of lens aberrations. Moreover there is a need tomaximize the lens region overlap of light diffracted from two, differentillumination shapes. Furthermore, there is a need and desire for a newmethod for determining the pitch of a second layer targets based on thepitch and light diffraction patterns of a first layer target thatminimizes displacement of the second layer targets by lens aberrationsdue to changes in illumination settings.

SUMMARY OF THE INVENTION

[0012] The invention relates to a method of determining a dimension fora semiconductor feature, in particular a second layer alignment target'spitch, to minimize the impact of lens aberrations during opticalprojection. In an exemplary embodiment, the design method determines thepitch of a second layer fine pattern alignment target based on the lightdiffraction patterns of a first layer fine pattern alignment target. Thefirst layer target is designed to have a pitch similar to that of aperiodic feature of the integrated circuit, such as a capacitor. Thesecond layer target is designed to have a pitch that minimizesdisplacement of the second layer target by optimizing the lightdiffraction patterns of the second layer target based on the first layertarget.

[0013] The pitch of the second layer target is determined by severalsteps. First, projection lens locations of the light diffractionpatterns created by the first layer target for a particular illuminationsetting are determined. Second, the projection lens locations of the0^(th) order light diffraction pattern for the second layer illuminationsettings are determined. Finally, the second layer target's pitch isselected which optimizes overlap of the 1^(st) order light diffractionpatterns of the second layer target with that of the 0^(th) orderdiffraction pattern of the first layer target. The more overlap betweenthe respective diffraction patterns of the first and second layertargets, the more the displacement error caused by lens aberrations isreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above issues and other advantages and features of theinvention will be more readily understood from the following detaileddescription of the invention provided in connection with theaccompanying drawings.

[0015]FIG. 1 is an illustration of a conventional optical imagingprojection system;

[0016]FIG. 2 is an illustration of conventional locations for electricalcircuit patterns and alignment targets for an integrated circuitfabricated on a semiconductor substrate;

[0017]FIG. 3 is a top view of a conventional box-in box target system;

[0018]FIG. 4 is a cross-sectional view of FIG. 3 along line III-III;

[0019]FIG. 5 is a top view of a conventional fine pattern target system;

[0020]FIG. 6 is cross-sectional view of FIG. 5 along line VI-VI;

[0021]FIG. 7 is a top view of a fine pattern target system according toan exemplary embodiment of the present invention;

[0022]FIG. 8 is a cross-sectional view of FIG. 7 along line VIII-VIII;

[0023]FIG. 9 is a cross-sectional illustration of an imaging system usedfor forming the first layer targets of FIGS. 7-8;

[0024]FIG. 10 is a cross-sectional illustration of the projection lenslocation of the 0^(th) order light diffraction pattern for the firstlayer targets of FIGS. 7-8;

[0025]FIG. 11 is a cross-sectional illustration of the projection lenslocation of the −1, 0^(th), and 1^(st) light diffraction patterns forthe first layer targets of FIGS. 7-8;

[0026]FIG. 12 is a cross-sectional illustration of an imaging systemused for forming the second layer targets of FIGS. 7-8;

[0027]FIG. 13 a cross-sectional illustration of the projection lenslocation of the 0^(th) order light diffraction pattern for the secondlayer targets of FIGS. 7-8;

[0028]FIG. 14 is a cross-sectional illustration of the projection lenslocation of the −1, 0^(th), and 1^(st) light diffraction pattern for thefirst layer targets of FIGS. 7-8 and the 0^(th) light diffraction ordersof the second layer targets of FIGS. 7-8;

[0029]FIG. 15 is a flow chart of the design method for determining thepitch for second layer targets of FIGS. 7-8 according to the presentinvention;

[0030]FIG. 16 is a cross-sectional illustration of the projection lenslocation for light diffraction patterns of the second layer targets ofFIGS. 7-8 which optimize the overlap of the 1^(st) order diffractionpattern of the second layer target with the 0^(th) order diffractionpattern of the first layer target according to the method of the presentinvention; and

[0031]FIG. 17 is a graphical bar chart comparison of the displacementerror for various aberration coefficients for a conventional finepattern target compared to a fine pattern target designed according to amethod of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] The present invention will be described as set forth in theexemplary embodiments illustrated in FIGS. 7-16. Other embodiments maybe utilized and structural and functional changes may be made withoutdeparting from the spirit or scope of the present invention.

[0033] FIGS. 7-8 are illustrations of an exemplary target system 600designed according to a method of the present invention as described inregards to FIGS. 9-16. FIG. 8 is a cross-sectional view of FIG. 7 alongline VIII-VIII. The fine pattern targets 610, 620 of first layer 615 areformed on layer 613. The targets 610, 620 are shown as two concentricgeometric squares. It is to be understood that other shapes, number oftargets, and arrangements are possible options, if so desired. The firstlayer targets 610, 620 are separated by distance P2, hereinafter calledpitch P2. Ideally, pitch P2 should be close to the circuit's limitingdimension, such as the spacing between capacitors in a DRAM device. Forexample, if the minimum device dimension is 1 micron, the first layertarget pitch P2 should be close to 1 micron as well.

[0034] In an exemplary embodiment, as described in FIGS. 9-16, the pitchP2 of target 610, 620, is selected at 1 micron to approximate thedimension of forming a trench in an insulating layer. The pitch P2 isthe distance from the perimeter 611 of the outside first layer target610 and the perimeter 612 of the inside first layer target 620. Thepitch P2 consists of the combined distance of line width W2 and spaceS2. The line width W2 and space S2 are selected at 0.5 microns each. Itis to be understood that the selection of P2, W2, and S2 can varywithout limiting the scope of the invention.

[0035] Referring to FIG. 8, a second layer 616 is shown formed overlayer 615 with second layer targets 630, 640. The pitch P3, line widthw3, and space width S3 of second layer targets 636, 640 are shown. Inthe exemplary design method of the present invention (as discussed inFIGS. 9-16), the pitch P3 is designed to minimize lens aberrations orimage displacement errors caused by projection lens 30. It is to beunderstood that the shape and number of fine patterns is not to belimited by the exemplary embodiment. Two concentric boxes made up ofthin lines are but one of numerous configurations for alignment targets.

[0036]FIG. 9 is a cross-sectional illustration of an annular lightsource 800 used to transfer an image of mask 810 containing first layertargets 610, 620 of FIGS. 7-8. The light source 800 of the exemplaryembodiment has a wavelength (λ2) of 248 nanometers. The outerillumination diameter 801 represented by line L2 is 0.8 numericalaperture units (N.A. units). The inner illumination diameter 802represented by L1 is 0.5 numerical aperture units. The image of mask 810is illuminated by light source 800 and reduced by projection lens 815before being transferred to a wafer (not shown). The projection lens 815has a numerical aperture 806 of 0.63 N.A. units. The lens axis 805 isshown by the center line.

[0037]FIG. 10 is a cross-sectional illustration of the projection lens815 location of the 0^(th) order light diffraction patterns 820generated by light source 800 passing through mask 810 containing firstlayer targets 610, 620 of FIGS. 7-8. The diffraction order locations forprojection lens 800 can be determined using an imaging software, such asSigma C, manufactured by Solid C Corporation of California, using knownlight diffraction equations. The location of the 0^(th) order region isrepresented by the two blocks 820. The 0^(th) diffraction orderrepresent regions where light was not deviated by the first layertargets 610, 620 of mask 810. Blocks 820 are located between 0.315 and0.501 N.A. units from the lens axis 805. The 0^(th) diffraction regionis 0.189 N.A. units wide. The two boundaries are established by lightfrom the outer 801 and inner 802 diameters of the light source 800passing through mask 810. The light paths are illustrated by lines 803,804.

[0038]FIG. 11 is a cross-sectional illustration of the projection lens805 location of the 0^(th) diffraction order 820, the 1^(st) diffractionorders 830, and the −1 diffraction orders 840 of first layer targets610, 620 formed on mask 810. Part of the 1^(st) diffraction order islocated between 0.067 and 0.256 N.A. units from the lens axis 805.Another part diffraction order is located at 0.563 and falls partiallyoutside the lens aperture. It is to be understood that the presentinvention can be used to analyze any diffraction order and is notlimited to the 0^(th) and 1^(st) order diffraction patterns.

[0039] The distance D1 between the boundaries of the 0^(th) orderdiffraction regions 820 and the 1^(st) order diffraction regions 830 isa function of the light wavelength λ2 and first layer target pitch P2.Distance D1 equals the wavelength λ2 divided by pitch P2:

D1=λ2/P2 or 248 nm/1000 nm=0.248 numerical aperture  (1)

[0040]FIG. 12 is a cross-sectional illustration of conventional lightsource 900 used to transfer an image of mask 910 containing second layertargets 630, 640 of FIGS. 7-8. The light source 900 of the exemplaryembodiment has a wavelength λ3 of 248 nanometers and diameter of 0.305microns. The diameter is represented by line L3 and the light source 900has an illumination diameter of 0.192 N.A. units. The image of mask 910is illuminated by light source 900 and is reduced by projection lens 815which was previously described above. Typically the same projection lens815 is used for the first 810 and second 910 masks.

[0041]FIG. 13 is a cross-sectional illustration of the projection lens815 location of the 0^(th) order light diffraction patterns 920generated by light source 900 passing through mask 910. The 0^(th)diffraction order locations for projection lens 815 can be determined asdiscussed previously. The location of the 0^(th) order region isrepresented by the block 920. Block 920 is between the lens axis 805 and0.192 N.A. units from the lens axis 805. The light paths are illustratedby lines 901, 902 travel from light source 900 through mask 910 toprojection lens 815. The lens location of the 0^(th) diffraction patternis independent of the pitch P3 of the second layer targets 630, 640.FIG. 14 is a cross-sectional illustration of the diffraction patterns820, 830 (FIG. 11) of first layer targets 610, 620 superimposed over the0^(th) diffraction 920 pattern of the second layer light source 900 asdiscussed in FIG. 13. The distance D2, which equals 0.507 N.A. units,represents the distance between the 0^(th) diffraction orders 820, 920of the first and second layer targets 610, 620, 630, 640.

[0042] To minimize the misalignment of the second layer targets 630, 640with the first layer targets 610, 620, the pitch P3 of the second layertargets 630, 640 should be designed to minimize lens aberrations. Anexemplary method for determining the pitch P3 for second layer targets630, 640 based on the light diffraction patterns of the first layertargets 610, 620 is described in FIG. 15 (process segments 520, 530,540, 550). The first segment 520 of the method 500 is to select a pitchP2 for first layer targets 610, 620. The second segment 530 is todetermine the projection lens locations of the diffraction ordersgenerated from first layer targets 610, 620 as described in FIGS. 10-11.The third segment 540 is-to determine the distance D2 between the firstlayer targets 610, 620 0^(th) diffraction orders 820 and the secondlayer targets 630, 640 0^(th) diffraction order 920. The distance D2 canbe solved using, for example, the graph in FIG. 14. The fourth segment550 is to select a pitch P3 for the second layer targets 630, 640 sothat the majority of the 1^(st) light diffraction order for the secondlayer targets 630, 640 passes through the projection lens 30 (FIG. 1) inthe region where the 0^(th) light diffraction order for the first layertargets 610, 620 passed.

[0043] An exemplary method is to select a pitch P3 whereby the distanceD3, between the 0^(th) diffraction order 920 and the 1^(st) diffractionorder 930, is equal to D2. It is to be understood that other methodscould result in more optimal overlaps, the above method does not limitthe scope of the invention. Other methods to include projection imagingsoftware or trial and error methods, could be used to analyzediffraction patterns for various pitches P3 to select pitch P3 forsecond layer targets 630, 640, which produce a 1^(st) diffraction order930 that maximizes overlap with the 0^(th) diffraction order 820 or the1^(st) diffraction order 830 of the first layer targets 610, 620. It isto be understood that the invention is not limited to this. For example,another method is to to maximize the overlap of the 0^(th) diffractionorder 920 of the second layer 616 with the 0^(th) diffraction order 820or 1^(st) diffraction order 830 of the first layer 615.

[0044]FIG. 16 is a cross sectional illustration of the exemplary methodof step 550 where distance D3 is selected to equal D2 which is 0.507(FIG. 14). The first diffraction order 930 of second layer targets 630,640 begins at 0.315 N.A. units from the lens axis 805 and extends pastthe lens numerical aperture 806. The distance D3 between the 0^(th)order diffraction pattern 920 and the 1^(st) order diffraction pattern930 is 0.507 N.A. units. Thus pitch P3 of target 630, 640 can be solvedusing the equation:

P3=(λ3/D3)=248 nanometers/0.507=0.489 microns  (2)

[0045] In most circumstances exact overlap will not be possible, sopitch P3 will be designed to minimize the amount of displacement errorfor a given illumination setting.

[0046]FIG. 17 is a bar graph comparison of the displacement errorsgenerated by lens aberrations for a conventional target system 200versus the target system 600 of the present invention. The bars labeled710 represent the lens displacement errors for prior art target systems200 for various light aberration coefficients. The bars labeled 720represent the lens displacement errors for target systems 600 formedaccording to the present invention. The use of various aberrationcoefficients to determine the refracting properties of a lens are knownin the art. FIG. 17 illustrates the impact that various aberrationcoefficients have on the displacement of the target systems 200, 600. Ataberrations coefficients 11 and 12, which correspond to comaaberrations, the displacement error for target system 200 exceeds 30nanometers, while the displacement error for target system 600 wasreduced by over 50%.

[0047] Having thus described in detail exemplary embodiments of theinvention, it is to be understood that the invention defined by theappended claims is not to be limited by particular details set forth inthe above description as many apparent variations thereof are possiblewithout departing from the spirit or scope of the invention.Accordingly, the above description and accompanying drawings are onlyillustrative of exemplary embodiments which can achieve the features andadvantages of the present invention. It is not intended that theinvention be limited to the embodiments shown and described in detailherein. The invention is only limited by the scope of the followingclaims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method for determining a pitch of a targetcomprising the steps of: determining light diffraction patterns of afirst layer target and selecting a pitch for a second layer target,wherein at least a portion of a 1^(st) order diffraction pattern of saidsecond layer target overlaps at least some portion of a 0^(th) orderdiffraction pattern of said first layer target.
 2. A method fordetermining a pitch of a second layer alignment target comprising thesteps of: determining light diffraction patterns of a first layeralignment target generated by a first illumination setting; determininga 0^(th) order light diffraction pattern generated by a secondillumination setting; determining a first distance between a 0^(th)order light diffraction of the second illumination setting and a 0^(th)order light diffraction pattern of the first layer alignment target; anddetermining a pitch for said second layer alignment target, wherein asecond distance between the 0^(th) order light diffraction patterns ofthe second illumination setting and a 1^(st) order diffraction patternfor the second layer alignment target generated by said pitch equalssaid first distance.
 3. A method of determining the pitch of a secondlayer feature on a mask, comprising the steps of: determining lightdiffraction patterns of a first layer feature on a mask and selecting apitch for the second layer feature, wherein at least a 1^(st) orderdiffraction pattern of the second layer feature overlaps at least someportion of a 0^(th) order diffraction pattern of the first layerfeature.
 4. A method of determining a pitch of a second layer feature ofa target comprising the steps of: determining a light diffractionpattern of a first layer feature for a first illumination setting;determining an n^(th) order light diffraction pattern for a secondillumination setting; determining a first distance between the n^(th)order light diffraction for the second illumination setting and anm^(th) order light diffraction pattern of the first layer feature; anddetermining a pitch for the second layer feature, wherein a seconddistance between the n^(th) order light diffraction pattern for thesecond illumination setting and the m^(th) order diffraction pattern forthe second layer feature equals said first distance.
 5. The method ofclaim 1, wherein at least one of said first layer and second layertargets is an alignment target.
 6. The method of claim 1, wherein saidat least one of said first layer and second layer targets is a finepattern alignment targets.
 7. The method of claim 1, wherein said firstlayer comprises at least two geometric shaped targets.
 8. The method ofclaim 1, wherein said second layer comprises at least two geometricshaped targets.
 9. The method of claim 1, wherein at least one of saidfirst layer targets comprise two targets, wherein a second target islocated inside the perimeter of the first target.
 10. The method ofclaim 1, wherein at least one of said second layer targets comprise twotargets, wherein a second target is located inside the perimeter of thefirst target.
 11. The method of claim 1, wherein at least one of saidtargets comprises a photoresist.
 12. The method of claim 7, wherein saidgeometric shapes comprise fine pattern squares.
 13. The method of claim8, wherein said geometric shapes comprise fine pattern squares.
 14. Amethod of forming two targets on at least two different layers of asubstrate, said method comprising: forming a first layer over thesubstrate; forming a first target in said first layer; forming a secondlayer over said first layer; and forming a second target in said secondlayer, wherein a pitch of said second target is determined by:determining light diffraction patterns of the first target and selectingthe pitch for the second layer target, wherein at least a 1^(st) orderdiffraction pattern of said second target overlaps at least some portionof a 0^(th) order diffraction pattern of said first target.
 15. Themethod of claim 14, wherein said targets are formed usingphotolithography techniques.
 16. The method of claim 14, wherein saidfirst layer is photomask.
 17. The method of claim 14, wherein saidsecond layer is a photomask.
 18. A target system comprising: a firsttarget formed in a first layer; and a second target formed in a secondlayer, wherein a pitch of said second target is formed by: determininglight diffraction patterns of the first target and selecting a pitch forthe second layer target and wherein at least a 1^(st) order diffractionpattern of said second target overlaps at least some portion of a 0^(th)order diffraction pattern of said first target.
 19. A projection imagingsystem comprising: an illumination source; a mask, said mask comprisingat least one target, wherein said target is formed by: determining lightdiffraction patterns of a lower layer target and selecting a pitch forthe mask target wherein at least a 1^(st) order diffraction pattern ofsaid mask target overlaps at least some portion of a 0^(th) orderdiffraction pattern of said lower target; a projection lens; and aprojection surface.
 20. The projection imaging system of claim 19,wherein the projection surface is a substrate.
 21. The projectionimaging system of claim 19, wherein the mask target are fine patterntargets.
 22. The projection imaging system of claim 19, wherein theprojection surface is coated with a photoresist material.
 23. Theprojection imaging system of claim 19, wherein the mask comprises andintegrated circuit fabrication pattern.
 24. A method for determining apitch for a second layer target comprising the steps of: determining thelight diffraction patterns of a first layer target and selecting a pitchfor said second layer target such that a predetermined order of a lightdiffraction pattern of said second target overlaps at least some portionof a predetermined different order light diffraction pattern of saidfirst layer target.